ad-asstra
Explanation
This comic features a conversation between an alien and a human about what makes humans unique in the cosmos.
The alien explains that humans are actually an amazing coincidence of size, placement, and behavior in the universe, but then adds an awkward caveat: "Well... you're the only creature in the cosmos that uses the anus for birth control."
The caption reads: "I began to regret inquiring about human uniqueness."
The joke plays on the common science fiction trope of humans asking aliens what makes humanity special among all intelligent life in the universe. Instead of receiving a flattering answer about human creativity, consciousness, or spirit, the human learns that humanity's distinguishing feature is an embarrassing biological/behavioral detail about anal sex as contraception.
The title "ad asstra" is a play on "ad astra" (Latin for "to the stars"), possibly with the extra 's' hinting at the anatomical subject of the punchline. The comic is characteristic of SMBC's tendency to deflate grand philosophical questions with crude but technically accurate observations about human nature.